Only 460hp

If the Mopar Performance heads are optimally ported for only .500" lift, there's a good chance that the valve springs that came with them are nowhere near what they need to be pressure-wize and you could be floating the valves, not to mention possibly having guide clearance issues and/or not enough installed height for the lift. Check for signs of damage.

Just my opinion, but for a street car:

* Get a 750 Holley DP carb & tune. Should be easier to tune and provide more air velocity/throttle response.

* SIR rods are rated for 500hp, which you're already shooting for. I figure the 4 speed and long stroke will be stressing the rods even more. Even less trust with nitrous. I'd suggest the Scat I-beams (A little bit heavier.) or H-beam rods even without nitrous.

* 1.5 rockers should be plenty.

* Is the cam installed/degreed properly.

* More compression will make a noticeable difference, if available gas can handle it